Synonyms: Haemoproteus maccallumi, Haemoproteus melopeliae, Haemoproteus turtur, Haemoproteus vilhenai (?).
Hosts: Domestic and wild pigeons, mourning dove, turtle dove and a large number of other wild columbiform birds. Levine and Kantor (1959) tabulated reports of Haemoproteus, all of which were probably H. columbae, from 45 species belonging to 19 genera of columbiform birds.
H. maccallumi was first described from the mourning dove (Zenaidura macroura). It is morphologically indistinguishable from H. columbae. Huff (1932) transmitted it from the mourning dove to the pigeon, but Coatney (1933) was unable to transmit it from the pigeon to the mourning dove; both used the hippoboscid fly, Pseudolynchia canariensis, as the vector. There may be strain differences between the different hosts, but until greater differences than these are brought out, it is probably better to use the name H. columbae for the species from columbiform birds.
Location: The gametocytes are in the erythrocytes. Schizogony occurs in the endothelial cells of the blood vessels.
Geographic Distribution: Worldwide.
Prevalence: Common. Thirty-eight reports of H. columbae from the domestic pigeon were tabulated by Levine and Kantor (1959). Among those in which relatively large numbers of birds were examined, Coatney (1935) found it in all of about 28 pigeons in midwestern United States, Kartman (1949) found it in 82% of 101 pigeons in the Honolulu zoo, Giovannoni (1945) found it in 58% of 159 pigeons in southern Curitiba, Brazil, Acton and Knowles (1914) found it in all of 75 pigeons from the plains of India, and Singh, Nair and David (1951) found it in 22% of 214 pigeons in Delhi, India.
Herman (1938) found it in 8% of 86 mourning doves (Zenaidura macroura carolinensis) on Cape Cod, Massachusetts, Huff (1939) found it in 47% of 188 mourning doves, mostly from Illinois, Couch (1952) found it in 56% of 213 mourning doves in Texas, and Hanson et al. (1957) found it in 30% of 392 immature and 43% of 72 adult mourning doves in Illinois; its incidence in this last survey increased steadily from 7 to 8% in very young doves to 70% in the oldest immatures, and varied markedly in different parts of the state and in different years. Wood and Herman (1943) found it in 93% of 27 western mourning doves in Arizona and California.
Morphology: The only stages found in the peripheral blood are macrogametes and microgametocytes. When mature, these are elongate and sausage-shaped. They partially encircle the host cell nucleus; they may displace it to some extent, but they do not push it to the edge of the host cell. They contain a variable number of dark brown pigment granules. The host cell is not enlarged.
When stained with a Romanowsky stain, the cytoplasm of the microgametocytes is pale blue or almost colorless and their nuclei are pale pink and diffuse, while the cytoplasm of the macrogametes is darker blue and their nuclei are compact and dark pink or red.
Life Cycle: The life cycle of H. columbae has been studied by Aragao (1908), Adie (1915, 1924) and Huff (1942) among others. Birds become infected when bitten by the dipteran vector. The sporozoites enter the blood stream and invade the endothelial cells of the blood vessels of the lungs, liver and spleen. Here they round up to form schizonts. Each schizont undergoes multiple fission to form 15 or more small, unpigmented bodies, the cytomeres, each with a single nucleus. Each cytomere grows still further, and its nucleus undergoes multiple fission. Finally, the host cell becomes considerably hypertrophied and is filled with a number of multinucleate cytomeres.
The endothelial cells break down, releasing the cytomeres. These vary in size, but may reach 60 u in diameter. They accumulate in the capillaries, which they sometimes block completely. They are irregularly shaped and tortuous, and may send out branches along the capillaries, becoming bifurcate, trifurcate or even multiradiate. Each cytomere produces an enormous number of merozoites, which break out and pass into the blood stream.
According to Wenyon (1926), the schizonts do not necessarily form cytomeres but may produce merozoites directly. Presumably, too, schizogony is repeated a number of times.
Following schizogony, the merozoites enter red blood cells and become macrogametes and microgametocytes. These first appear 28 to 30 days after infection. At first they resemble ring stages of Plasmodium, but grow to the mature, elongate form in a few days. Multiple infections of erythrocytes with immature parasites are not uncommon, sometimes as many as 12 being found in a single host cell, but infections with more than 1 mature gamete or gametocyte are rare.
The only proven vector is the hippoboscid fly, Pseudolynchia canariensis (syns., Lynchia maura, L. lividicolor, L. capensis). In addition, Araglio (1916) stated that Microlynchia pusilla is a vector in South America, but gave no experimental evidence. Baker (1957) found that H. columbae from the English wood pigeon (Columba palumbus) would undergo sporogony in Ornithomyia avicularia, but 6 attempts to infect domestic pigeons by bite or injection of infected louse-flies failed.
It is highly unlikely that hippoboscids are the only vectors of this species, however. As Hanson et al. (1957) pointed out, hippoboscids are extremely rare on mourning doves, yet H. columbae is common in them. The discovery by Fallis and Wood (1957) that biting midges (Culicoides) are the vectors of H. nettionis of ducks suggests that they may also transmit H. columbae.
In the stomach of the hippoboscid vector, the microgametocytes produce 4 or more snake-like microgametes by exflagellation. They fertilize the macrogametes, and the resultant zygotes are ookinetes which crawl to the midgut wall and form oocysts on its outer surface. These grow, reaching a diameter of about 36 u. They become mature in 10 to 12 days, producing very large numbers of slender, falciform sporozoites up to 10 u long and similar to those of Plasmodium. These break out of the oocysts into the body cavity and pass to the salivary glands, where they accumulate and are injected into a new host when the fly bites it.
Pathogenesis: H. columbae is only slightly pathogenic. Infected birds usually show no signs of disease. In relatively heavy infections the birds may appear restless and go off feed, and anemia may result from destruction of erythrocytes, but this is unusual. The liver and spleen of affected birds may be enlarged and dark with pigment.